The invention relates to an agent for producing non-metallic coatings on iron or steel, based upon an alkali-silicate-polyphosphate sol with a content of inorganic pigment; and to a method for producing non-metallic coatings on iron or steel.
Various methods are known for applying nonmetallic coatings to metal surfaces, especially for purposes of electrical insulation. In addition to phosphating processes, especially those based upon alkaline-earth phosphates, in which the solution is baked-on at a high temperature (German Pat. Nos. 10 30 377, 10 90 916), there are processes in common use which produce heat-resistant coatings by applying to the metal surface aqueous sodium-silicate solutions and a metal oxide, e.g. cobalt and/or nickel oxide; the coating is dried and then baked-on (U.S. Pat. No. 2,711,974).
U.S. Pat. Nos. 3,213,302 and 3,562,011 suggest compositions useful for imparting electrical resistance but neither recognizes the advantage obtained by employing an alkali water-glass, polyphosphate and insoluble chromic oxide pigment.
The disadvantage of known methods using water-glass is that the coatings thereby produced pick up moisture. The consequence of this is a considerable impairment of insulating action and mechanical properties, such as adhesion.
One known agent and method for producing nonmetallic layers on sheet-iron and steel, which is intended to overcome these known disadvantages, consists of an alkali-silicate-polyphosphate sol with specific molar ratios as regards silicate, alkali-oxide and polyphosphate, the application of which is described in German Disclosure Text 27 25 619. The sol may also contain, among other things, inorganic oxygen-containing boron compounds, saponifiable polyacrylate polymers, inorganic pigments and/or inorganic fillers. The pigments used are iron-oxide or lithopone, the fillers mica or calcium carbonate.
Although the non-metallic layers produced by the foregoing methods have satisfactory resistance to moisture, they have the disadvantage of unsatisfactory electrical resistnce after high-temperature annealing (usually above 750.degree. C.), such as is usually carried out for the purpose of improving the magnetic properties of stampings.
It is the purpose of the invention to provide an agent and a method for producing non-metallic coatings on iron or steel which lacks known disadvantages, especially those mentioned above, which can be produced without high cost and can be used to form layers.